1. Field of the Invention
The invention relates to the field of serial transceivers, and in particular, to speed negotiation between transceivers having different characteristics.
2. Background Information
A number of different speeds and standards are currently used and being developed in serial communications systems, for example, fiber optical communication links. Serializer/Deserializer (SerDes) modules are used in digital communications system transceivers to serialize and deserialize data which is sent in serial form over a communications medium.
Existing transceivers, also called legacy transceivers, have generally been designed to operate at a different and lower speed than the newly emerging transceivers. It is desirable to be able to continue to use the legacy transceivers in existing systems, but also be able to connect them with other systems having the faster emerging transceivers. A user may want to upgrade a system to the faster transceivers in stages, and therefore it would be desirable to be able to continue using the legacy transceivers while upgrading.
However, currently, problems may be encountered in a link having two or more transceivers designed for different operating speed standards. Generally, a link having transceivers with different operating speeds can operate consistently only at the speed of the lowest speed transceiver. Therefore, the higher speed devices need to be able to operate at the lower speed.
Further, it would be advantageous if each transceiver could automatically detect the respective speeds of the other transceiver to which it is connected in a point-to-point link, automatically determine when a speed adjustment is necessary, and automatically make the speed adjustment.
One proposal which was discussed at a meeting of the Fibre Channel Optical Working Group in the Fall of 1999, was to encode and detect different baud rates using a lights on, lights off sequence, the rate of xe2x80x98flashingxe2x80x99 being related to the baud rate. For example, a 1 Hz flashing rate would mean a 1 GHz baud rate. However, so far no standard has been adopted in this regard.
Therefore, a need exists for an arrangement to implement an automatic baud rate negotiation between transceivers having different baud rate operating characteristics.
Along with automatic baud rate detection and adjustment, there is also a need to select and adjust to the appropriate speed-dependent (baud rate dependent) filtering because different link baud rate/speed standards require different transceiver bandwidths. Solutions to the bandwidth adjustment problem are described in copending application attorney docket ROC920000029 (IBM-141), application Ser. No. 09/574,239, filed May 19, 2000, entitled xe2x80x9cSWITCHABLE-BANDWIDTH OPTICAL RECEIVERxe2x80x9d and assigned to the same assignee as the present application. A number of different ways in which receiver bandwidth can be adjusted are disclosed therein.
Known Fibre Channel standard optical transceivers have control input/output signals (I/O""s) known as Transmit Disable (TxDsbl) and Receive Loss of Signal (RxLOS). These signals are presently used to disable a transmission and indicate the loss of a received signal, respectively. These signals may be used diagnostically to detect and/or indicate problems in a communications link, or for power conservation reasons, for example.
It is, therefore, a principle object of this invention to provide a method and apparatus for speed (baud rate) negotiation for serial transceivers.
It is another object of the invention to provide a method and apparatus that solves the above mentioned problems so that existing serial transceivers can continue to be used with emerging transceivers having different operating characteristics.
These and other objects of the present invention are accomplished by the method and apparatus disclosed herein.
According to an aspect of the invention, an arrangement to enable speed (baud rate) negotiation between transceivers having different operating speed characteristics is implemented using existing optical transceiver control input/output signals. In particular, Fibre Channel Transmit Disable (TxDsbl) and Receive Loss of Signal (RxLOS), together with serial data input into the transceiver from Serializer/Deserializer (SerDes) modules, are used to create an event which will allow the transceiver to detect the baud rate of the signal that the SerDes is transmitting, for example.
According to another aspect of the invention, when an RxLOS is detected on either end of the link, one of the possibilities is that a new port has been plugged in and the data transmitted from that port might not be speed-compatible with the receiver. If any event that might cause a change in port speed occurs, e.g., fiber swapping, transceiver swapping, electrical switching of serial streams, the TxDsbl input can be cycled on and off for a long enough period of time to induce a RxLOS on the receiving end.
According to another aspect of the invention, RxLOS on the receiving end can also trigger the same cycling of TxDsbl on that end so that both ends of the point-to-point link receive a RxLOS.
According to another aspect of the invention, TxDsbl can be followed by the lowest frequency clock like data pattern that is desired to be transmitted by the transceivers. This way, the transceivers can default to their lowest bandwidth filtering and still be able to decipher the pattern.
According to another aspect of the invention, in Fibre Channel, a K28.7 code, or binary 10 bit equivalent (0011111000), is preferred.
According to another aspect of the invention, if the K28.7 pattern is used, this algorithm can be extended across a 5xc3x97 change in baud rate since the lowest bandwidth receiver can still decipher the 0011111000 pattern if multiplied up by 5 to a 0101010101.
According to another aspect of the invention, the sending of the K28.7 pattern is multiplexed with data into the Serializer/Deserializer (Ser/Des) Transceiver, selected by a logical processing of the RxLOS signal. This logical process will work with duplex or simplex cabling and tolerate a single lane being disconnected at a time.
According to another aspect of the invention, after the transceiver detects that a signal is present, it will start discriminating the baud rate.
According to another aspect of the invention, frequency discrimination is done in the transceiver using a reference oscillator signal and a simple up counter which counts the number of transitions in a given oscillator period to decode the baud rate.
According to another aspect of the invention, the number of bits required in the up counter is proportional to the number of transitions per oscillator period.
According to another aspect of the invention, with respect to the Fibre Channel standard, the invention can be implemented in the physical layer with some buffering logic to control the RxLOS and TxDsbl.
According to another aspect of the invention, the problem of selecting the appropriate baud rate dependent filtering is solved by using a specific training pattern during events that could cause transceiver modules of different speed characteristics to be connected. Advantageously, this means that there is no additional pin required from an existing form factor device to signal the filtering bandwidth required for operation. The detection of the baud rate is done on the transceiver itself.
These and other aspects of the invention will become apparent from the detailed description set forth below.